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DRAFT - November 9, 1999 MEMORANDUM TO: PUBLIC SERVICES DRAFT FROM: MIKE CALLAHAN SUBJECT: QX 99-2 The Flood Control Division has concerns regarding the western area of phase II of the quarry project due to the proximity of the excavation to the Mokelumne River. As proposed, the project would be inundated by floodwaters from the Mokelumne River during a 100-year flood event. In addition, the project as proposed would require a State Reclamation Board permit prior to excavation. The 100-year flood elevation at the site is 85.7. An appropriate setback from the Mokelumne River needs to be established to mitigate for the flood hazard. In developing the setback standard, the following should be considered: 1. Once excavation has occurred, the remaining land subject to flooding adjacent to the quarry would act as a levee. Therefore, levee standards regarding top width, side slopes and freeboard should apply. In this case, the standards would require a top width of 20 feet, a 2:1 landside slope and three feet of freeboard. 2. Although explicit detailing of potential conditions of a State Reclamation Board Permit is not possible, the State Standards generally require a setback of 500 feet. 3. Some areas of the Mokelumne River are subject to extreme erosion. A setback from potential erosion of the river bank should be established. It is recommended that the following conditions be applied to the project: 1. The applicant shall apply for a State Reclamation Board Permit and all conditions of that permit shall be followed. 2. A geotechnical evaluation shall be performed by a registered professional which establishes the potential for riverine erosion and determines an appropriate setback. 3. A minimum setback of 20 feet from a surveyed line determined by elevation 88.7 shall be required for all areas of the project where excavation occurs within 500 feet of the banks of the Mokelumne River.